Interpretive Summary: Dietary energy restriction (ER) remains a cornerstone of most approaches to long-term weight loss, and current recommendations promote the use of moderate energy deficits of 500–1,000 kcal/day below the amount required for weight maintenance. This deficit translates to a reduction in energy intake of typically 20–40% for an individual with an energy requirement of 2,500 kcal/day and is estimated to result in weight loss of 1–2 lb/week. Moderate levels of ER are recommended on the grounds that greater degrees of ER do not achieve better long-term weight loss. We describe here an analysis of data from the first phase of the Comprehensive Assessment of the Long-term Effects of Restricting Intake of Energy (CALERIE) study at Tufts, testing the hypothesis that individuals randomized to very low (10%) ER lose less body weight and fat over 1 year than individuals randomized to moderate (30%) ER.
The results of our study suggest that prescribing a low-level (10%) ER may result in a mean actual ER and weight loss over 1 year comparable to values obtained from prescribing a moderate (30%) ER. We also observed for the first time that individual success with a low ER prescription was significantly more variable than that with a moderate ER.
These findings combined with previous related research suggest that 10–20% reductions in energy intake may on average be as effective for achieving long-term weight loss as higher levels of restriction in some individuals but not others, and suggest avenues for future research that may lead to optimization of weight loss programs based on individualizing the prescribed degree of ER.

Technical Abstract:
Theoretical calculations suggest that small daily reductions in energy intake can cumulatively lead to substantial weight loss, but experimental data to support these calculations are lacking. We conducted a 1-year randomized controlled pilot study of low (10%) or moderate (30%) energy restriction (ER) with diets differing in glycemic load in 38 overweight adults (mean +/- s.d., age 35 +/- 6 years; BMI 27.6 +/- 1.4 kg/m2). Food was provided for 6 months and self-selected for 6 additional months. Measurements included body weight, resting metabolic rate (RMR), adherence to the ER prescription assessed using 2H2 18O, satiety, and eating behavior variables. The 10% ER group consumed significantly less energy (by 2H2 18O) than prescribed over 12 months (18.1 +/- 9.8%ER, P = 0.04), while the 30% ER group consumed significantly more (23.1 +/- 8.7%ER, P < 0.001). Changes in body weight, satiety, and other variables were
not significantly different between groups. However, during self-selected eating (6–12 months) variability in percent weight change was significantly greater in the 10% ER group (P < 0.001) and poorer weight outcome on 10% ER was predicted by higher baseline BMI and greater disinhibition (P < 0.0001; adj R2 = 0.71). Weight loss at 12 months was not significantly different between groups prescribed 10 or 30% ER, supporting the efficacy of low ER recommendations.
However, long-term weight change was more variable on 10% ER and weight change in this group was predicted by body size and eating behavior. These preliminary results indicate beneficial effects of low-level ER for some but not all individuals in a weight control program, and suggest testable approaches for optimizing dieting success based on individualizing prescribed level of ER.